Length matters! Within reason, if you double the length of an HF mobile antenna, you increase the radiation resistance by a factor of four. The ratio is the same if you're shortening the antenna. Plus, the shorter the antenna is, the more inductance it takes to resonant it. This causes the coil resistive losses to increase at a much higher rate than the change in radiation resistance. As a result, a 5 foot antenna will have an efficiency about 1/6th that of a ten foot one.
To directly answer your question, the MFJ is about as good as an ATAS 120. Yes, you'll make a few contacts, and you might even be happy with it. One thing's for sure, driving around with a dummy load for an antenna will either sour your feel for HF mobile, or make you yearn for more.